Conventional commercial dyeing with indigo is well known. According to one conventional indigo dyeing method, a sheet of yarns is dyed by sequentially dipping (with a high wet pick-up of about 65%-75%) in leuco-indigo in several indigo dye vats allowing air to oxidize the dye on the yarns after each dip. Another conventional indigo dyeing method involves a series of ropes of yarns each containing about 400 individual yarns that are dyed in a series of indigo dye vats in a similar manner. The sheet dyeing method generally also includes a step of applying sizing to the yarns in preparation for weaving. The rope dyeing method has the disadvantage that the individual dyed ropes must be opened (re-beaming) so the yarns in the rope can be combined at sizing to make the necessary number of ends for a full width warp. These additional steps are self-evidently labor intensive and problematic due to yarn breakages.
The conventional rope dyeing method allows for continuous operation as one rope can be tied to the tail of another. The sheet dyeing method on the other hand must be stopped and reset with full input beams which thereby results in substantial waste and potential color changes from one lot of yarns to another.
Conventional indigo dyeing methods employ large amounts (e.g., on the order of several thousands of liters) of dye bath which, due to cost and environmental concerns must be stored for the next use. By way of example, a commercial indigo dye house might have several baths stored in preparation for future use. When needed the entire dye range must be emptied and refilled with the appropriate bath or the existing bath must be adjusted in concentration. This changeover time thereby reduces the efficiency of the ranges and reduces quality because the leuco-indigo dye tends to oxidize over time in storage.
The conventional indigo dyeing methods are also relatively slow, e.g., generally operating in the range of 20-35 meters per minute, and use excessive volumes of water and chemical additives. By way of example, in order to maintain the condition of the leuco dye, reducer and caustic additives are typically added to the dye vat so as to prevent the buildup of oxidized indigo dye that will contaminate the system and the yarn being dyed. Overflow frequently results from the volumes of chemical additives. The machinery needed for conventional indigo dye systems are also energy intensive due to the required yarn drying and the needed horsepower to pull large quantities of wet yarn.
There has been a trend in the market for beams from the rope system (after opening or re-beaming) to be threaded to various take-up mechanisms to wind each yarn back into cones. Such yarns are used in accent stripes, in the weft of various fabrics, or in knits. The demand for such indigo dyed yarns recovered to cones is increasing. Both conventional indigo dyeing systems are directed toward high volume and are thereby not conducive to rapidly changing customer demands for fashion, especially stretch yarns in warp direction, which are harder to process in either of the conventional systems.
It has also been suggested that conventional indigo dyeing processes are not environmentally sustainable due to the large amounts of water and energy that are consumed in addition to the use of chemical additives, such as reducers and caustic agents which generate salts and high alkalinity in the wastewater. Such wastewater is typically capable of being neutralized using sulfuric acid at conventional waste treatment facilities. But in many underdeveloped countries, little or no wastewater treatment is available, resulting in environmental contamination.
Indigo dyeing methods are also known that involve low wet pick up techniques; for example, the use of indigo dye foam and aerosol spray. However, these techniques face the additional problem of high exposure to oxygen due the surface area of the bubble (inside and out) or to the surface area of the aerosol droplets. For this reason nitrogen is used to create an inert atmosphere.
For example, U.S. Pat. No. 8,215,138 (the entire contents of which are incorporated expressly hereinto by reference) describes the benefits of using nitrogen in a sealed container over the dye bath and further teaches the use of a dwell chamber. U.S. Pat. Nos. 8,167,958, and 7,913,524 (the entire contents of each such patent being expressly incorporated hereinto by reference) propose low wet pick-up methods and require a reduction in the oxygen present further suggesting a nitrogen medium, however, neither patent specifies what level of oxygen contamination they require in each element of the system, nor do they propose to measure the actual oxygen content in the system. None of these known systems has demonstrated the success required to replace conventional sheet or rope indigo dyeing. An overview of conventional indigo dyeing can be found in the literature, for example, in Vuorema, Anne, Reduction and Analysis Methods of Indigo, ISBN978-951-29-3781-3, Turun Ylioopiston Julkaisuja Annales Universitatis Turkuensis (2008), the entire content of which is expressly incorporated hereinto by reference.
An indigo dyeing process and apparatus for natural yarns and fabrics that could solve the deficiencies in conventional indigo dyeing systems as discussed above, as well as the previously proposed low wet pick up processes, would be of great utility, especially in countries where water is scarce and/or energy is expensive. It is towards providing such solutions that the embodiments of the herein disclosed invention are directed.